Modern magnetic tape drives employ combination read/write heads that incorporate multiple side-by-side data heads along with one or more read heads dedicated to sensing servo tracks added to, or interleaved with the data tracks. As the density of the data tracks is increased from 500 tracks per inch to 5000 tracks per inch, the accuracy of the servo system must increase to maintain proper alignment. If the size of the servo tracks is not decreased, then valuable surface area on the tape is not available for data storage, and the head becomes more complicated due to the unequal spacing of the magnetic heads. Reducing the size of the servo tracks frees more space on the tape, but makes it more difficult for the servo system to maintain the necessary positional accuracy. Dedicated servo tracks can be eliminated all together using a complex process of encoding the track and data together when writing, and signal processing to extract a position error signal when reading. Even when this is done, differences in the format of the servo information from tape drive type to tape drive type can make a tape written on one machine unreadable on another.
Several approaches have been developed to uncouple the function of magnetic data storage from the function of servo tracking by using optics. Early approaches used optical edge sensors to follow one or both edges of the tape. Using these edges as references, the data tracks were defined at fixed distances across the tape. These approaches were subject to tracking misalignment as the edges of the tape became worn and tattered. They also did not transfer well to disk media where the inner data tracks were several inches away from the disk edge requiring precision offsets over long distances.
To eliminate the dependency of the condition of the tape edges, optical servo tracks were placed on the back side of the tape. While this improved tracking accuracy, this method is difficult to carry out in small form factors because access is required on both sides to the tape. Further more, establishing initial mechanical alignment from tape drive to tape drive, and maintaining that alignment for several years is difficult with the optical tracking components on one side of the tape and the magnetic data components on the other side. To overcome this alignment problem, optical marks have been added to the magnetic head to calibrate the optical tracking components. This however increases the size and complexity of the total system.
In mid-1970's developments in the field of optical disks produced optical tracking systems using a single narrow laser beam to read and track data recorded on the disks. An example of such a system is disclosed in U.S. Pat. No. 4,057,833 issued to Braat on Nov. 8, 1977. The system taught by Braat can follow an optical track of data only 0.5 .mu.m wide. Modern optical disk systems can support up to 40,000 tracks per inch to accuracies of better than 0.1 .mu.m. A couple of patents have been issued which merge the single beam servo tracking capabilities from the optical disks with magnetic disk media. U.S. Pat. No. 4,558,383 issued to Johnson on Dec. 10, 1985 discloses an information storage disk transducer position control system using prerecorded optical servo patterns. In Johnson, a magnetic head is mounted for common movement with an optical head that follows reflective patterns above or below the magnetic coating. U.S. Pat. No. 4,958,245 issued to Roth et al. on Sep. 18, 1990 discloses an apparatus and method that employ an optical servo system for use with a magnetic head and a magnetic medium. Roth et al. discloses the use of a bi-cell or a quad-cell optical detector to follow the edge of optical servo tracks cut into the magnetic side of the medium. The optical head is then mechanically attached to the magnetic head at a fixed distance to align the magnetic head with the magnetic data tracks. However, the prior art does not disclose the accuracy and reliability obtained by an array of servo track heads, combined with the fine servo track widths of the optical heads, and integrated with the magnetic heads into a single combination head for stability.